A. Field of the Invention
This invention relates generally to packaging, and more particularly to a container for storing stacks of planar, charge-storing materials.
B. Description of the Related Art
Recording media and graphic arts products such as lithographic printing plates often take the form of planar laminates, and may include one or more metal layers sandwiched between dielectric material (such as polymer sheets). As such laminate media are handled and prepared for packaging, their dynamic motion through air can generate sufficient friction to produce an electrostatic charge on the exposed dielectric layer or layers. That charge accumulates and is stored on the metal layers as the laminates are stacked, forming a multi-level charged capacitor. Depending on the dimensions of the laminates, the accumulated charge and corresponding voltage can become significant. Consequently, personnel handling the stacked materials can be exposed to discomfort or even physical danger if their actions result in simultaneous contact with two or more metal plates.
Dissipation of electrostatic charge from packaged goods has long been a concern in the semiconductor industry, where charge-sensitive devices can be damaged during handling, transit and storage unless appropriate measures are taken. These measures typically involve the design of special shipping containers that prevent voltage buildup by conducting charge from the terminals of devices packed within the containers to the exteriors thereof. Any excess charge either circulates until it decays to a harmless value or bleeds off to ground when the containers are placed on grounded surfaces.
Because such containers are designed for small electrical components, the necessary electrical connections can be established by inserting the components' terminals into conductive foam that is itself electrically coupled to conductive exterior surfaces of the container. Such a configuration is unsuitable for many types of stacked laminates, however, since charge dissipation from such articles requires electrical contact with the conductive layers thereof; it is impossible to achieve the intimate contact afforded by mechanical insertion.
Exacerbating the difficulty of establishing a good electrical connection between stacked graphic-arts laminates and a container is the fact that such laminates often consist of layers that are quite thin, so that the edges of conductive layers cannot provide adequate surface area to establish strong electrical contact. This problem becomes even more acute with respect to laminates that contain extremely thin aluminum layers, such as spark-imaged lithographic printing plates. Although such layers can store significant charge if their surface areas are large, their edges are prone to formation of a nonconductive oxide layer. This further reduces the electrical effect of whatever contact the edges of such thin layers can make with opposing surfaces of the walls. Accordingly, traditional charge-dissipating container designs that might be modified to make contact with the edges of stacked articles will not prove suitable for graphic-arts applications.